Process of treating steel



Patented May is, 1943 UNITED STATESPATENT OFFICE PROCESS OF TREATING STEEL Wilbur A. Saylor, Bellevue, Pa.

No Drawing. Application September 30, 1941,

Serial No. 413,043

4 Claims. (c1. 75-129) out the use of lead in the analysis and the difliculties which are attendant, naturally, upon the addition of lead to a steel.

For a number of years steels have been classified as normal or abnormal, depending upon the extent to which the grains are surrounded by the grain boundarymaterial, i. e., cementite L or ferrite. When the grain boundary material is well defined and substantially continuous around each grain, the steel is said to be normal, but when the grain boundary material is not substantially continuous, the steel is said to be present possibility of the operators becoming lead-poisoned. Additionally, the large quantities of lead employed in industrial applications abnormal, and the degree of abnormality depends upon the extent to' which the boundary material is interrupted. A steel is classified as normal or grade A steel when it has a substantially continuous boundary of cementite or ferrite around each grain. Abnormal steels are classified in the trade (Dodge test) in grades B, C and D. In grade B, the boundary is slightly broken; in grade C 'it is considerably broken; and in grade D, it is more or less limited to one or possibly two sides of the grain.

For approximately the same length of time the steel industry has classified the sizes of grains in steel according to the standard chart of the Americal Society for Testing Materials. This chart extends from No. 1 for very coarse grains, to No. 8 for very fine grains.

The present invention relates particularly to.

normal steels which are classifiable in grade A, which have grains ranging in size between about Nos. 1 and 4 on the A. S. T. M. Standard scale, and which, although free from lead, have a machinability comparable to that of leaded steel.

Machinability is a somewhat controversial term to define; it may refer to the surface produced, to the number of pieces. of work finished in a unit of time, to the-tool life, or to chip formation. In the present description, machinability will be understood to embrace all of these factors. That is to say, in accordance with the present invention, the foregoing factors are balanced properly to produce more of a satisfactory product than has been possible heretofore.

It is well known that free-machining steels are produced frequently by adding lead in various amounts, such additions producing steels which machine at high production rates and with a long tool life.

The use of lead in steels has certain objections. First, there may be mentioned the eversuch as the main component of bearing metal, electric cable sheaths, pigments, and a multitude of other uses place lead in the category of a strategic metal.

These, among other reasons, make the elimination of lead highly desirable.

In accordance with the present invention, there are provided steels of widely different composition in which there has been included sufficient nitrogen to render the addition of lead unnecessary. Such steel may be made by either the Bessemer process, or open hearth, and should have a composition within the following approximate ranges:

Bessemer Open hearth Per cent 0. 05 0. 40

Some silicon, copper, nickel, molybdenum in accidental or residual amounts may be present in the composition-without materially influencing the machining properties of the improved steel of the present invention.

In accordance with the present invention, the

nitrogen is introduced into the steel by using ani hydrous ammonium sulphate which is mixed in the steel in the ladle.

The use of anhydrous ammonium sulphate possesses definite advantages over other nitrogen-bearing compounds because it avoids the intense combustionwhich results if inflammable materials, such as urea, for example, be employed; also it avoids the toxic hazard of cyanides, the carburizing action of organic compounds, and the contaminating action of metallic nitrogen salts. The low cost of ammonium sulphate also affords a definite economic advantage, since in any event a large excess of the nitrogen-bearing material must be employed in order to obtain the requisite nitrogen content in the metal. Thus, using ammonium sulphate as the nitrogen-bearing compound there should be i used enough to contain about four times the.

hearth furnace are tapped without addition of term-manganese or ferro-silicon to the furnace prior to tapping, and little or no aluminum is added to the ladle. In any event, either a me- TYPE or STEEL SAEX-1112 N'o'r- NITROGEN TREATED chanically capped or semi-killed steel is em- 5 ployed, and any aluminum used to deoxidize is T001 life added in amounts only suflicient to form some M11 P S N aluminum oxide, but not enough to combine with Minutes any of the nitrogen.

A further advantage of the use of ammonium l0 6-066 3 sulphate resides in the introduction of sulphur 0111 1107 0:106 0332 01011 8. as in the steel along with the nitrogen, the sulphur also enhancing the free machining of the steel, NITROGEN TREATED up to the point of rendering the steel hot-short.

Since insufiicient sulphur may be present in the 15 0.10 0.88 0.108 0. 292 0.01:; 55 steel even after the addition of ferro-manganese, ferro-silicon and ammonium sulphate for intro- 8 27 ducing nitrogen into the steel, any deficiencies in (L M1 0099 M88 M15 9 I the sulphur content may be made up by adding 9 21 elemental sulphur to the steel in the ladle. 81 3; 8: g 23 It is observed also that the desirable properties 5 0 par d by the n r n e enh n d by ph Leaded Sm], Standard analyses Z 23 phorus, appreciable beneficiation beginning with a Phospmms abmlt 010%, and From the foregoing, it will be seen that the tendmg through the range of from 9 25 nitrogen-bearing Bessemer stee1 containing from to about Undqr normal practlce com .013% nitrogen to about 0.016% nitrogen gives drawing, very beneficial effects are obtained at very Satisfactory tool life, and is in fact phosphorus parable to leaded steel in machining qualities. ms phosphorus content 15 present normally This improvement is noted in nitrogen contents in Bessemer steels; but in the case of open hearth up to approximately 0 020% steels, rephosphorizing, with ferro-phosphorus added to the ladle may be employed to bring the EXAMPLE 2 phlosphorus content in the metal to its preferred Machinability test run on standard part open va ue.

The improved effects of the present invention healrth steel are illustrated by the following data, wherein the Revolut p minute- 2110 333 2110 steels referred to have the following normal comeed .0055" .0055" .0070" position range: No. of pcs. per hour 818 857 974 w ssh: aar test. ets

SAE 1120 lain 355 Fair Wouldnotnm.

Open hearth 0 Mn P s N .18 .82 .014 .142 .003 SAE 1120 open hear 5'30" Fair 1'40" F i do Plus N 0 Mn P s N v SAE uz'o i h lfifi 7'55" Goo 5'45" Good.

Plus N and P 0 Mn P s N SAl 1 l20 11 1115 6'05 Fair 0'00" Fair.

0 Mn 11 120 in 1% SAE udiinis ieshqsnln- 5'20" Good.

N given.

SAEX-1112 as a Bessemer steel From the foregoing, it will be seen that an open C 008% 0 16% hearth steel containing 0.012% of phosphorus -""1"? 0'600/ and a like percentage of nitrogen showed den- 1 6Q nite improvements over the same steel in which 8 f 6 26 the nitrogen was substantially absent.

However, when the steel was phosphorizedjp .S'AE-1120 as an open hearth steel bring the phosphorus content up to 0.047%, and a nitrogen content of 0.01% substantially better C 0'15% 0'25% results were obtained. It will be seen from the Mn foregoing data that with 974 pieces'machined per P 045% maximum hour with 2,110 revolutions per minute and a cut S of 0.007" in depth, the tool would not run on the EXAMPLE 1 steel containing 0.012% phosphorus and 0.012% Machinability test run on standard part cold g ggfi g gg gg g ii ggig gfgg $22 23? spwmens tained with an increase of phosphorus to 0.047%. Revolutions per minute 2110 2333 On the other hand, the open hearth steel con- Feed (depth of cut, inches) 0.007 0.0085 taining 0.012% nitrogen behaved substantially. No. pieces per hr..-- 974 1200 better than did the same steel with nitrogen substantlally absent, the tool life for the nitrogencontaining steel being shown as five hours and 80 minutes with 818 pieces per hour machined at a depth cut of 0.0055" and 2110 revolutions per minute, whereas the steel which was substantially free from nitrogen giving only a tool life of three hours and 55 minutes, under these same conditions.

When the output was increased to 857 pieces per hour, the nitrogen-bearing steel gave a tool life of one hour and 45 minutes while this same steel with a phosphorus content of 0.047% gave a tool life of seven hours and 55 minutes.

The same steel with low phosphorus and substantially nitrogen-free but with a lead content of 0.16% which is the customary lead content of free-machining steels gave only a tool life of six hours and 5 minutes, with an output of 857 pieces per hour and this leaded steel showed substantially the same tool life at an output of 974 pieces per hour as did the higher phosphorus steel contaming nitrogen under the same conditions. The surface finish of the nitrogen-bearing steel containing phosphorus of 0.047% gave a better surface finish than did the low phosphorus substantially nitrogen-free steel containing the lead.

The explanation of the improved results obtained by the addition of nitrogen to high-phosphorus steels is not known, and there is no intention nor desire to be bound by any explanatory theory. However, it may be that the nitrogen increases machinability because of the possibility that the nitrogen may dissolve in the ferrite and afiect the plasticity of the ferrite. The steels as processed in accordance with the present invention are machined usually in the colddr'awn condition and a large amount of mechanically produced slip has occurred already before the machine tool acts upon the steel. The machining may embrittle the ferrite for there is a certain amount of cold work done ahead of the cutting tool by action of the tool, causing a precipitation of nitrides to occur and, therefore, providing a steel making a more brittle chip. It has been observed that when the nitrogen exceeds a maximum value the machining properties deteriorate. v

I claim:

1. The process of treating steel to improve machinability thereof which comprises adding sulphur and nitrogen to molten steel contained in a ladle by incorporating in the steel ammonium sulphate in sufficient quantities to contain approximately 4 times the amount of nitrogen that is to be included in the steel.

2. The process oftreating steel to improve machinability thereof which comprises adding ammonium sulphate to molten steel contained in a ladle in quantities sufficient to impart a nitrogen content to the steel of not more than approximately 0;020% but more than 0.003%.

' 3. The process of improving machinability of steel which comprises adding ammonium sulphate to molten steel in quantities sumcient to impart a nitrogen content to the steel of not more than 0.020% but more than 0.003%, the'said nitrogen being imparted to the steel in the presence of from about 0.010% to about 0.2% phosphorus.

. 4. The process of improving machinability of steel which comprises adding ammonium sulphate to molten steel in quantities sufficient to impart to the steel -up to about 0.020% nitrogen but more than 0.003%, the said nitrogen being imparted to the steel in the presence of about 0.010% to about 0.2% phosphorus and from about 0.1% to about 0.300% of sulphur.

WILBUR A. SAYLOR. 

